As the granulation method in a granulation step of forming granules, for example, of urea, and as the method of coating granules, many proposals have been made. For example, the applicant of the present application has proposed a method disclosed in JP-B ("JP-B" means examined Japanese patent publication) No. 63729/1992 as a method for processing large particles of urea. A case of this granulation method is described using a granulator illustrated in FIG. 4, by way of example.
FIG. 4 is a vertical cross-sectional view of a granulator. In this figure, seed crystals of urea are fed into a granulation section 1 through a feed port 2. Molten urea liquid is sprayed at a prescribed angle from nozzles 3 into the granulation section 1. As a result, the above seed crystals of urea are subjected to the spray of molten urea in the granulation section 1 and grow in granule diameter. The grown urea 7 is whirled up into a space 6 by currents for jetting from multiple air feed pipes 5 branched from a lower feed port 4, and the urea 7 is allowed to drop into a space 8. Meanwhile, a fluid for the fluidization is fed from an upper feed port 9, to keep the grown granular urea 7 on a bottom bed 10 in a fluidized state in the space 8, so that the granular urea is fluidized to occupy throughout the space 8 over the nozzles 3. This movement is repeated, and finally the granular urea is discharged from a discharge port 11.
However, in this granulator, sometimes, due to some reason during the continuous operation, the molten urea jetted from the nozzles 3 provided on a line 12, or the grown granular urea 7, adheres to some of outlets 13 of the air feed pipes 5, to prevent the air from being jetted out and to clog the outlets. As a result of the occurrence of the malfunctioned section, the fluidized state in the space 8 becomes turbulent, the granulation